This invention relates generally to the thermoplastic elastomers and in particular, to the thermoplastic elastomers which have been thermally oriented.
Thermoplastic elastomers possess properties of both thermoplastics and elastomers and have a wide range of applications. In certain applications, a thermoplastic elastomer film is dimensionally stabilized in a stretched condition (wherein stresses and strains in the film have been frozen in) for subsequent use. For example, the stretched thermoplastic elastomer may be applied to a substrate and later heated causing the thermoplastic elastomer to shrink and retain substantial elastic properties. One such use is disclosed in European patent application No. 84301717.9 (Publication No. 0119825) wherein the stretched dimensionally stable thermoplastic elastomer is placed on a diaper waistband and reheated causing the thermoplastic elastomer to contract and revert to a heat stable elastic state. The diaper thus, is provided with a flexible and stretchable waistband. A similar application of thermoplastic elastomers is disclosed in European patent application No. 84301720.38 (Publication No. 0119827). These publications are cited merely to disclose a possible use of thermoplastic elastomers.
In many applications, particularly where the thermoplastic elastomer is secured to a substrate for later contraction by the application of heat, it is important that the thermoplastic elastomer have relatively high shrink force since the substrate resists shrinkage. The shrink force is determined by measuring the shrinkage of the film sample against an applied force and is referred to herein as weighted shrink. The weighted shrink properties differ markedly from free shrinkage (no applied weight) and hence is a key property in determining the suitability of a heat shrinkable film on substrates.
U.S. Pat. No. 4,303,571, issued to D. S. Jansen et al disclose a thermoplastic elastomer film comprising 25 to 55 parts by weight of an ethylene-propylene copolymer, 35 to 55 parts by weight of an EVA copolymer and 15 to 25 parts by weight of a liquid hydrocarbon process oil.
As will be demonstrated in the discussion of the comparative examples contained herein, the elastomeric film made from the composition disclosed in U.S. Pat. No. 4,303,571 possesses satisfactory shrink properties, but not nearly as good as those possessed by the composition of the present invention, particularly with respect to weighted shrink properties.
A problem associated with thermoplastic elastomers is tackiness which impedes unwinding of the film during processing. This is a particularly serious problem with film compositions containing EVA and processing oil. Moreover, addition of particulate antiblocks are not particularly effective in thermoplastic elastomers because of the difficulty in dispersing the additives uniformly on the film surface, since the additives tend to concentrate in the elastomer phase.